How do i allow 3 moving game object of same tag to be attached to each other upon collision?

Hi guys, I hope you are fine! I am super new in Unity, the basic idea of this game I am working on is: all objects on the conveyors are moving in the direction indicated by the arrow, the player needs to drag objects of same colour next to each other, once a combination of 3 obj of same colours is detected, a green circle appears around them and only then can the player click on the coresponding container to get point. The only part i'm stuck at is how to make the combination i mentioned above, make appear the green circle and allow the tank to detected that combo to allocate point and maintain distance between the objects. For now all I managed to do is moving objects and getting points when one obj is dragged unto the right container. Your help would be very much appreciated! Thank you alt text